Optical arrangement

ABSTRACT

A solid-state laser or arrangement for wavelength conversion is disclosed, which in its simplest embodiment is comprised of a light-generating body arranged in a supporting means, the light-generating body having a shape which is substantially complementary to a guiding structure which is formed in the supporting means. The guiding structure is formed with a high degree of accuracy, for instance, by etching the supporting means or by replicating an original. Between the light-generating body and the guiding structure of the supporting means a thin contact layer is arranged, the purpose of which is to increase the adherence to and/or the heat transfer to the supporting means. Due to the fact that the contact layer is a deformable material, possible discrepancies as regards complementary between the guiding structure and the light-generating body will be filled by the contact layer whereby a close fit is obtained between the complementary structure.

TECHNICAL FIELD

The present invention relates to a light-generating arrangement whichcomprises at least one supporting means in which a light-generatingsolid body is arranged, and to a method for arranging optical componentsin a supporting means.

PRIOR ART

An arrangement for generating coherent light in the form of asolid-state laser consists in its simplest embodiment of alight-generating, solid body of a host material (e.g. glass or crystal)doped with active ions (e.g. rare earth metals or transition metals) anda set of mirrors which define an oscillator cavity round thelight-generating body. The light-generating body is usually suppliedwith the requisite energy optically, so-called optical pumping. Suchpumping is usually effected by means of a flash lamp or a laser diodearrangement.

Another arrangement for generating coherent light is a so-called opticalparametric oscillator (OPO). In an OPO the light-generating bodycomprises a non-linear crystal. An OPO does not generate light in thesame way as a laser, but functions as a converter from one lightwavelength to another. The pumping of an OPO is carried out by pumpingcoherent light, which has been generated in some other way, into theOPO, whereby coherent light is generated at other wavelengths. Otherarrangements for wavelength conversion function in a similar way.Examples of different types of wavelength conversion are frequencydoubling, sum frequency generation, difference frequency generation andparametric frequency generation.

A holding device for a solid laser body is previously known from, forinstance, DE-196 43 531 A 1. This holder uses a foil between the laserbody and the holder for increasing the heat transfer from the laser bodyto the holder. Recesses are made in the holder for receiving superfluousfoil material, for instance, in connection with expansion of the laserbody. Thus, the aim is to reduce internal stress in the laser body.

U.S. Pat. No. 5,265,113 discloses an integrated microsystem forelectronic and optical components which are mounted in anisotropicallyetched structures in a base plate of semiconductor material. Thecomponents are electrically controllable and movable in such a mannerthat their positions relative to the base plate are actively adjustable.

The above-mentioned arrangements for generating coherent light have,however, limitations as regards the light power which can be extractedfrom a light source of a certain volume. In connection with opticalpumping of coherent light sources, the efficiency is lower than 100%.This means that some of the energy which is deposited in thelight-generating body of the arrangement is lost in the form of heat.Since arrangements for generating coherent light, which are based onsolid, light-generating bodies, such as solid-state lasers andarrangements for frequency conversion, are driven towards higher andhigher output power, problems often arise as regards the heat transferfrom the light-generating body because of the low heat conductivity ofthe dielectric materials of which the light-generating body typically iscomprised. The heating which thus arises results in thermal expansion ofthe heated area and other thermal effects, such as thermal lensing,thermal birefringence and reduced gain (caused by, inter alia, adecrease of the lifetime of the excited state in the active ions of thelaser material and/or an increased thermal population of the energylevels of the active ions).

Furthermore, there is a risk that, for instance, the laser material of asolid-state laser cracks as a result of thermally induced internalstress. This is a particularly great problem as regards laser materialwith an anisotropic atomic structure.

For the non-linear crystal in an OPO or some other frequency-convertingarrangement, the main problems consist of thermal lensing and poor phasematching caused by the heating.

The miniaturisation of coherent light sources of the type mentionedabove involves a reduction of the volume which is occupied by the lightbeams in the light-generating body. In order not to increase the thermalloading, the power of the light source has to be decreased. This is aconsiderable limitation of prior-art technique since both aminiaturisation of the light source and an increase of the output powerusually are desired in one and the same light source.

The problems mentioned above have limited the output power fromminiaturised, coherent light sources to be typically a few hundredmilliwatt.

DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a construction foroptically pumpable light sources by means of which the handling of thethermal loading on the light-generating body as well as the alignment ofcomponents included in the light source are simplified to a considerableextent.

Another object of the invention is to provide a manufacturing methodwhich allows mass manufacturing of optically pumpable light sourcesaccording to the present invention and which allows simple incorporationof further components in the arrangement. Examples of further componentswhich advantageously are incorporated in the arrangement are so-calledfunctional elements. A functional element should, as regards the presentapplication, comprise non-linear crystals for frequency conversion,active or passive Q-switches for producing light pulses with high peakpower, as well as active or passive mode locking means to produceultrashort light pulses. The term “functional element” also comprisesmeans for external intensity modulation or phase modulation and meansfor controlling the direction of the emitted light beam.

According to the invention, a solid-state laser or an arrangement forwavelength conversion is thus provided which in its simplest embodimentis characterised by an optically pumpable, light-generating body of adielectric material, the body being arranged in a supporting means andhaving a shape which is substantially complementary to a guidingstructure formed in the supporting means. The guiding structure isformed with a high degree of accuracy, for instance, by etching thesupporting means or by replicating an original. Between thelight-generating body and the guiding structure of the supporting means,a thin contact layer is arranged, the purpose of which is to improve theadherence to and/or the heat transfer to the supporting means. Due tothe fact that the contact layer consists of a deformable material, anydiscrepancies as regards complementarity between the guiding structureand the light-generating body will be filled by the contact layer,whereby a close fit is obtained between said complementary structures.Preferably, the contact layer has a thickness less than 100 micrometer.A thickness of some tens of micrometers is especially preferred.

Examples of optically pumpable, light-generating solid bodies ofdielectric material which might constitute the light-generating body ofthe invention, are optically non-linear crystals for frequencyconversion and laser material based on crystal or glass.

According to another aspect of the invention, a method for manufacturingthe above-mentioned light sources is provided. Briefly, the method ischaracterised in that a plate of a crystalline material (a supportingmeans) is provided with one or more guiding structures which have ashape that is substantially complementary to the light-generating body.Alternatively, a supporting means is provided with guiding structures byreplicating an original. In the guiding structure a contact layer of thetype mentioned above is arranged, preferably by vapour deposition,electroplating or sputtering of a jointing metal, after which thelight-generating body is arranged in the guiding structure of thesupporting means. According to a preferred embodiment, the supportingmeans comprises at least two parts which jointly enclose the major partof the area of the side faces of the light-generating body, while twoopposite end faces of said body are let free for the passing of light.The supporting means is conveniently mounted in thermal contact with athermoelement with the aim of allowing control of the temperature of thelight-generating body. The supporting means may also be provided withmicrochannels for further increase of the possibility of thermalcontrol.

One advantage of the present invention is that the light-generating bodyfills the entire guiding structure. This embodiment gives together withsaid contact layer an excellent heat transfer from the light-generatingbody to the supporting means.

Another advantage of the invention is that, thanks to the supportingmeans being micromechanically provided with well-defined guidingstructures, the device can with a high degree of accuracy beminiaturised to millimetre dimensions. The limitation of the heattransfer from the light-generating body is usually the limited thermalconductivity of the light-generating body. A reduction of itscross-sectional area thus results in a substantial reduction of thethermal loading on the arrangement. The lower limit of the cross-sectionis determined by the cross-section of the light beam which propagatesthrough the arrangement.

The term “miniaturised light source” relates mainly to bulk lasers andarrangements for frequency conversion, whose optically pumpablelight-generating body has a cross-sectional area, perpendicular to thepropagation direction of the light, which is in the range of less than 1mm² to a few mm², but the invention also relates to otherlight-generating bodies, for instance, waveguiding structures.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention will be more evident whenreading the following detailed description of a number of preferredembodiments in connection with the accompanying drawings, in which

FIG. 1a is a cross-sectional view of a preferred embodiment of anarrangement according to the invention,

FIG. 1b is a schematic perspective view of the embodiment shown in FIG.1a, one of the parts of the supporting means being removed,

FIG. 2 is a schematic side view of an embodiment, where several elementsare arranged in the supporting means,

FIG. 3 is a schematic side view of an embodiment where the supportingmeans is comprised of two parts which peripherally enclose the elementsarranged in the supporting means,

FIG. 4 is a schematic top plan view of yet another embodiment of theinvention, and

FIG. 5 is a schematic illustration of a method according to theinvention.

PREFERRED EMBODIMENTS

FIG. 1a is a cross-sectional view, transversely to the intendedpropagation direction of the light, of a preferred embodiment of thepresent invention. FIG. 1b is a perspective view of the embodiment shownin FIG. 1a, one of the enclosing parts of the arrangement being removed.

According to this embodiment, the optically pumpable, light-generatingarrangement 101 is a laser arrangement which comprises a supportingmeans 103, 106 provided with a guiding structure 104. The supportingmeans 103, 106 comprises a first silicon plate 103 (the supportingsurface of which is constituted by the <100> plane of the siliconcrystal) which, by an anisotropic etching method well-known to thoseskilled in the art, is provided with the guiding structured 104. Thisguiding structure 104 consists of a v-grove, the side faces of which areconstituted by the <111> plane of the silicon crystal, the bottom angle121 being 70.5 degrees.

In the guiding structure 104, a contact layer 105 is arranged whichconsists of a relatively soft jointing metal. By the expression “softmetal” is here meant a plastically deformable metal which has a hardness(Brinell) below 1000 MPa (preferably below 100 MPa), or alternatively issofter than a laser material 102 which is arranged in the guidingstructure 104. Especially preferred jointing metals are indium and tin(hardness 9 and 51 MPa, respectively), or suitable solderable alloysthereof (e.g. InSn, InAg, PbSn, AgSn). As an alternative to a jointingmetal a heat conducting adhesive may be used.

The laser material 102 (i.e. the optically pumpable, light-generatingsolid body of dielectric material) is precision-sawn in such a mannerthat its side faces 108 are substantially complementary to the guidingstructure 104 in which the laser material 102 is arranged. The end faces107 of the laser material are conveniently provided with dielectricmirrors (e.g. multiple-layer coatings, not shown in the figure) whichdefine a laser cavity. The supporting means 103, 106 also comprises asecond silicon plate 106 which is identical to the first plate 103 andserves as a “lid” over the laser material 102.

The arrangement thus comprises a laser material 102 which is arrangedbetween two silicon plates 103, 106 with etched guiding grooves 104, thelaser material 102 being in contact with the guiding grooves 104 via acontact layer 105 consisting of a soft metal, the contact layer 105 thusbeing arranged between the laser material 102 and the guiding groove104. According to this embodiment, the laser material 102 is rhomboid incross-section. When the two silicon plates 103, 106 provided withguiding grooves are pressed together round the laser material 102, anexcellent fit to the guiding groove 104 is obtained thanks to the shapeof the laser material, which is complementary to the guiding groove 104and the soft jointing metal. The rhomboid shape of the laser material102 also contributes to the force components 131 which act on the lasercrystal 102 being symmetrically oriented, which further increases thestability and the fit to the supporting means 103, 106 and, thus, thethermal contact between the laser material 102 and the supporting means103, 106. A light propagation path 120 has been indicated.

The contact layer 105 has a thickness of some tens of micrometers andthe jointing metal preferably has good thermal conductivity with a viewto increasing the heat transfer between the laser material 102 and thesupporting means 103, 106. Examples of suitable laser materials aredielectric crystals, such as yttrium-aluminium-garnet (YAG),yttrium-lithium-fluoride (YLF), yttrium vanadate (YVO), yttrium-orthoaluminate (YALO), doped with, for instance, rare earth metals ortransition metals, but also other solid-state materials may be suitable,such as doped glass.

According to an alternative embodiment, the laser material is triangularin cross-section, the “lid” of the supporting means being substantiallyflat.

As shown in FIGS. 2-4, further alternative embodiments may be obtainedby forming guiding structures 204, 304 in the supporting means 203, 303,306, 403 and arranging one or more functional elements and/or lens meanstherein in connection with the laser cavity. The functional elements canbe placed inside the cavity as well as outside (after) the same. Theterm “functional element” here comprises components which in some wayaffect the properties of the emitted light, such as non-linear crystalsfor frequency conversion, active or passive Q-switches to produce lightpulses with high peak power and active or passive mode locking means toproduce ultrashort light pulses. Also elements for external intensitymodulation or phase modulation and/or control of the direction of thelaser beam are comprised.

FIGS. 2-4 schematically show optically pumped arrangements according tothe present invention. In the shown cases optical pumping is effected bymeans of an optical fiber 212, 312, 412. The light emitted from thefiber is focused into the laser material 202, 302, 402 through suitablelens means 210, 310, 410, for instance, conventional lenses,graded-index lenses (GRIN lenses) or non-imaging lens ducts.

FIG. 5 shows an example of a method for arranging an optical component502 in a supporting means 503. The method comprises providing a plate ofan etchable material, preferably silicon, with a guiding groove (guidingstructure) 504, in which the optical component 502 can be arranged. Thisguiding structure 504 is conveniently produced by anisotropic wetetching with potassium hydroxide, ethylene diamine-pyrocatechol ortetramethyl-ammonium hydroxide after masking the supporting means 503photolithographically to define the guiding groove 504.

A jointing metal as described above is then deposited over thesupporting means 503 by, for instance, vapour deposition, sputtering orelectroplating. The depositing may take place through a mask, either ashadow mask or a photo-resist mask, if depositing is desired only overcertain parts of the supporting means 503.

An optical component (an optically pumpable, light-generating material)is precision-sawn in order to obtain a body 502, the side faces 508 ofwhich are substantially complementary to the guiding structure 504. Thelight-generating body 502 is then arranged in the guiding structure 504,an excellent fit with the supporting means 503 being obtained thanks tothe well-defined guiding groove 504. The adhesion to the contact layer505 can be improved by coating the precision-sawn side faces 508 of thebody with a metal layer which also serves as a reflecting layer forlight that propagates inside the light-generating body 502.

Preferably, but not necessarily, the light-generating body 502 is givena rhomboid shape and the guiding structure 504 is given a v-shape. A lid(106 in FIG. 1a) is preferably arranged over the arrangement in order toobtain an enclosing supporting means (103, 106 in FIG. 1a) whichperipherally encloses the light-generating body (102 in FIG. 1a). Aperipherally enclosing supporting means gives, when in operation, thearrangement a considerably more symmetric temperature profile. Also therhomboid shape contributes to a more symmetric temperature profile,which has been shown by simulations using finite element methods.

Any optical coatings on the end faces 507 of the light-generating bodyare conveniently deposited on an original plate before it isprecision-sawn to the selected shape. When sawing, the plate isprotected from sawdust by a protective film.

According to an alternative method, deep dry etching is used instead ofthe above-mentioned wet etching. By this method grooves with anarbitrary geometry can be formed.

According to another method, guiding structures are produced byreplicating an original, which has been formed in the above-mentionedmanner (possibly with an inverted geometric shape). The originalpreferably consists of silicon since the possibility of microstructuringis considerable as regards silicon. The replication is, for example,made by electroplating of thick (>100 micrometer) metal layers on thesilicon surface. The obtained metal plate is then separated from thesilicon plate and, thus, constitutes a perfect negative image of theoriginal. If the original is not geometrically inverted relative to theintended shape of the supporting means, the obtained metal plate will,in its turn, be replicated to provide the correct geometric shape of thesupporting means.

In all the described embodiments, the supporting means canadvantageously be provided with microchannels for transporting coolingfluid, thereby obtaining an increased possibility of controlling thetemperature of the arrangement. The supporting means is convenientlyarranged in thermal contact with a thermoelement (for example a Peltierelement) for a further possibility of temperature control. The use ofmicrochannels with cooling fluid may then be avoided.

In other embodiments of the invention, a plurality of light-generatingbodies are placed side by side to provide a light source with an arrayof light sources. In such an embodiment of the invention the outputpower from the arrangement can easily be escalated to high power levels.Moreover, the cross-sectional profile of the emitted beam can becontrolled by individual control of the sources included in the array.

Although the invention has been described by means of embodiments wherethe light-generating body is a laser material, the invention alsorelates to other light-generating materials, such as non-linearcrystals. A common feature of the materials in question is that they areoptically pumpable, dielectric. Furthermore, there are no limitations asregards the use of silicon in the supporting means, nor any limitationsas regards guiding structures that are formed as v-grooves. On thecontrary, in several applications it will be advantageous to selectother materials for the supporting means and/or other geometries for theguiding grooves. In the described embodiments, the optical pumping iscarried out by means of an optical fiber. Alternative pump sources whichmay be possible in various embodiments comprise laser diodes and laserdiode arrays.

What is claimed is:
 1. An arrangement for generating light, comprising an optically pumpable, light-generating solid body of a dielectric material having a contact surface contacting a supporting means, wherein the contact surface of the light-generating body contacting the supporting means is substantially complementary to a guiding structure which is formed in the supporting means, a contact layer is arranged substantially between the contact surface and the guiding structure, the contact layer being comprised of a deformable material, the supporting means comprises, at least partly, an etchable, crystalline material, and the supporting means comprises, at least partly, an etchable, crystalline material of silicon, and wherein the guiding structures are constituted by, along the <111> plane of the silicon crystal, etched v-grooves with a bottom angle of 70.5 degrees, the contact surface of the light-generating body contacting the supporting means having a triangular or rhomboid shape.
 2. An arrangement as claimed in claim 1, wherein the contact layer is softer than the light-generating body and has a thickness less than 100 micrometer.
 3. An arrangement as claimed in claim 2, wherein the thickness of the contact layer is less than 30 micrometer.
 4. An arrangement as claimed in claim 1, wherein the contact layer comprises indium.
 5. An arrangement as claimed in claim 1, wherein the contact layer comprises a thermally conductive adhesive.
 6. An arrangement as claimed in claim 1, wherein the contact layer comprises a solderable alloy.
 7. An arrangement as claimed in claim 1, wherein the light-generating body is an optically pumpable laser material.
 8. An arrangement as claimed in claim 1, wherein the supporting means comprises at least two parts which together peripherally enclose the light-generating body, while at least one propagation path through the light-generating body is open for propagation of light through the arrangement, the light-generating body substantially filling the entire guiding structure which is formed in the supporting means.
 9. An arrangement as claimed in claim 7, wherein the optically pumpable laser material is arranged to be supplied with energy via an optical fiber.
 10. An arrangement as claimed in claim 1, wherein a functional element is arranged in connection with the light-generating body.
 11. An arrangement as claimed in claim 1, wherein the light-generating body is a non-linear crystal.
 12. An arrangement as claimed in claim 11, wherein the supporting means comprises at least two parts which together peripherally enclose the non-linear crystal, while at least one propagation path through the nonlinear crystal is open for propagation of light through the arrangement.
 13. An arrangement as claimed in claim 1, wherein the supporting means is provided with microchannels for transporting cooling fluid through the supporting means.
 14. An arrangement as claimed in claim 1, the arrangement also comprising a thermoelement with which the supporting means is in thermal contact.
 15. An arrangement as claimed in claim 1, wherein a plurality of optically pumpable, light-generating bodies are arranged side by side to provide an arrangement for generating light by an array of light sources.
 16. A method for arranging an optically pumpable, light-generating solid body of a dielectric material in a supporting means of light-generating arrangements, the method comprising the steps of: providing at least one guiding structure by etching in an original that is at least partly crystalline, providing at least one guiding structure in the supporting means by replicating the original, the guiding structure being given a shape which is substantially complementary to a surface of the optical component, the surface being intended for contact with the guiding structure, the guiding structure being given a shape which is substantially complementary to a surface of the optical component, the surface being intended for contact with the guiding structure, applying a contact layer over substantially the entire contact surface between the guiding structure and the optical component, the contact layer being comprised of a deformable material that is softer than the optical component, and having a thickness less than 100 micrometer, and arranging the optical component in the guiding structure of the supporting means so that substantially the entire surface, complementary to the guiding structure, of the optical component is in contact with the guiding structure via the contact layer.
 17. A method as claimed in claim 16, further comprising the step of coating the surface, complementary to the guiding structure, of the optically pumpable solid body with a reflecting layer.
 18. A method as claimed in claim 16, wherein the contact layer between the optically pumpable solid body and the guiding structure is applied by vapour deposition of a plastically deformable, thermally conductive material.
 19. A method as claimed in claim 16, further comprising the step of providing the supporting means with microchannels for transporting a cooling fluid.
 20. A method of using an arrangement as claimed in claim 1, wherein an optical component of glass or dielectric crystal is arranged in a guiding structure of a supporting means, and a deformable contact layer is provided between the optical component and the guiding structure of the supporting means. 